The present invention relates to a cross-linkable or cross-linked rubber composition which is usable to constitute a tire tread having improved wear resistance, to such a tread and to a tire incorporating this tread. The invention applies in particular to tires of passenger-vehicle type.
Since fuel economies and the need to preserve the environment have become priorities, it has become desirable to produce mixes having good mechanical properties and as low a hysteresis as possible so that they can be processed in the form of rubber compositions usable for the manufacture of various semi-finished products involved in the constitution of tires, such as treads, and in order to obtain tires having reduced rolling resistance.
Among the numerous solutions proposed for reducing the hysteresis of tread compositions and, consequently, the rolling resistance of tires comprising such compositions, mention may, for example, be made of the compositions described in U.S. Patent Specification Nos. U.S. Pat. No. 4,550,142 and U.S. Pat. No. 5,001,196; EP-A-299 074 or EP-A-447 066.
An attempt has also been made to improve the grip of the tires using tread rubber compositions incorporating specific plasticizers.
European Patent Specification No. EP-A-1 028 130 thus describes a rubber composition for a tire tread which is intended to improve the grip of the tire incorporating it. This rubber composition comprises a polymeric resin obtained by copolymerization of dicyclopentadiene and limonene. The units resulting from the polymerization of limonene may be present equally well in a minority or majority quantity in this resin, as shown by the examples of embodiment of this document in which the resins have been obtained with mass fractions of 32% limonene to 68% dicyclopentadiene (examples 1 to 9) or alternatively 67% limonene to 33% dicyclopentadiene (example 10).
European Patent Specification Nos. EP-A-1 063 246, EP-A-1 029 873, EP-A-990 669 and EP-A-1 077 223 also describe rubber compositions for tire treads which are intended to improve the grip of the tires incorporating them. These rubber compositions each comprise a polymeric resin obtained by copolymerization of four monomers consisting of dicyclopentadiene or of dimethyl-dicyclopentadiene, limonene, a polycyclic aromatic hydrocarbon (indene) and a monocyclic aromatic hydrocarbon (alkyl styrene or vinyl toluene). In all the examples of embodiment of each of these documents, the respective mass fractions of the aforementioned four monomers are either 25%, 25%, 25%, 25% or 12.5%, 27.5%, 25%, 25%.
In addition to this reduction in the rolling resistance and this improvement in grip, it is equally desirable to improve the wear resistance of the tire treads and, consequently, to increase the life of the latter (this improved wear resistance also having the effect of reducing over time the debris of tires on the ground due to traveling and the quantity of worn tires which are sent for recycling, which helps to preserve the environment).
Relatively few solutions have been proposed to date to improve this wear resistance. Mention may be made, for example, of the compositions described in Patent Specification JP-A-61 238501, EP-A-502 728 or EP-A-501,227.
Now, it is well-known to the person skilled in the art that an improvement in one performance type for tires is frequently obtained to the detriment of the other performance types. By way of example, mention may be made of the use in tread compositions of amorphous or semi-crystalline polymers having a high glass transition temperature (Tg) or melting temperature and a reduced molecular weight, the effect of such use being to improve the grip on dry or damp ground of the corresponding tires but also to adversely affect their wear resistance.